A large number of publications have appeared relating to the aim and purpose of the use of powdered rubbers and possible processes for their preparation.
The explanation for the interest in pulverulent rubbers can readily be found from the processing techniques of the rubber industry. There, rubber mixtures are prepared with a high expenditure of time, energy and personnel. The main reason for this is that the raw material rubber takes the form of balls, and the other constituents of the vulcanizable mixture must be incorporated.
Comminution of the balls and intimate mixing with fillers, mineral oil plasticizers and vulcanization auxiliaries takes place on mills in internal mixers in several process stages. Between the stages, the mixture is generally cooled on a batch off-line, deposited on pallets as rolled sheets and intermediately stored. The internal mixers or mills are followed by appropriate extruders or calendering processes.
Only a completely new processing technology can lead away from this very involved technique of rubber processing.
The use of free-flowing rubber powders has therefore been considered for some time, because of the possibility of being able to process rubber mixtures easily and quickly in the same way as thermoplastic powders.
DE-PS 2822 148 discloses a process for the preparation of a pulverulent, filler-containing rubber.
According to this patent specification, an aqueous filler emulsion is added to a rubber latex (e.g. natural rubber) a rubber solution (e. g. BR) or an aqueous emulsion of a synthetic rubber (e. g. SBR) and the desired rubber powder is precipitated out.
To avoid the particle size-dependent filler contents obtained by this process, patents have been applied for relating to variants, for example DE-PS 15 3723 213 and DE-PS 3723 214.
DE-PS 3723213, discloses a process which proceeds in two stages. In the first step, an amount of .gtoreq.50% of the filler is integrated into the rubber powder particles. In the second step, the remainder of the filler is absorbed on to the so-called rubber base particles. This can be regarded as a variant of powdering, since no bond is formed between the filler and rubber.
As E. T. Italiaander (Paper 151. Technische Tagung der Rubber Div der ACS [Technical Conference of the Rubber Div. of the ACS], Anaheim, Calif. May 6-9 , 1997 (GAK 6/1997 (50) 456-464) found, however, regardless of the great future predicted in the Delphi Report (Delphi Report "Kunftige Herstellverfahren in der Gummiindustrie [Future Production Processes in the Rubber Industry]" Rubber Journal, vol. 154, no. 11, 20-(1972)) for pulverulent and granulated rubber, and despite numerous attempts undertaken by well-known polymer producers from the mid 1970's into the early 1980's to prepare pulverulent NBR, SBR-carbon black masterbatches and granulated NR, the standard delivery form of polymers has remained rubber balls.
One disadvantage of the known processes lies in the fact that a grinding operation is required in order to establish the particle diameter of the filler particles of 10 .mu.m regarded as necessary for the quality of the end product. This not only results in a high expenditure of energy, but also causes damage to the filler structure, which, along with the active surface area, is an important characteristic parameter for the effectiveness in use in rubbers.
Another disadvantage is that the ease of handling of the products according to the prior art suffers due to the particles sticking to one another during storage.
A precipitation process in which suspensions of fillers modified with organosilicon compounds are prepared and stirred into the rubber emulsion is the subject of German Patent application P 198 16 972.8. The rubber powder is then precipitated out of this mixture.